Wind turbines typically include a rotor with large blades driven by the wind. The rotor blades convert the kinetic energy of the wind into rotational mechanical energy. The mechanical energy usually drives one or more generators to produce electrical power. Thus, wind turbines include a power transmission system to process and convert the rotational mechanical energy into electrical energy. The power transmission system is sometimes referred to as the “power train” of the wind turbine. The portion of a power transmission system from the wind turbine rotor to the generator is referred to as the drive train.
Oftentimes it is necessary to increase the rotational speed of the wind turbine rotor to the speed required by the generator(s). This is accomplished by a gearbox between the wind turbine rotor and generator. Thus, the gearbox forms part of the drive train and converts a low-speed, high-torque input from the wind turbine rotor into a lower-torque, higher-speed output for the generator. Although gearboxes are used in many industries, there are particular challenges in designing them for wind turbines due to the magnitude, variety, and unpredictability of forces experienced by the wind turbine rotor and drive train. These forces have the potential to damage bearings and other gearbox components. As a result, gearbox reliability has traditionally been a concern in the wind power industry.
Some manufacturers address this concern by designing power transmission systems without a gear stage. The wind turbine rotor directly drives a low-speed generator in such systems. Although there may be no concerns about gearbox reliability, the lack of a gear stage often gives rise to other concerns. In particular, the low-speed generators in direct-drive wind turbines are typically larger than their high and medium-speed counterparts in geared solutions to produce equivalent amounts of power. The larger size presents transportation, assembly, and maintenance challenges in addition to cost concerns. Moreover, many of the low-speed generators rely upon permanent magnets incorporating rare earth materials of limited availability.
The competing concerns between traditional drive trains and direct-drive machines has led to increased interest in medium-speed solutions. These “hybrid” solutions typically include an integrated gearbox and medium-speed generator. One such solution is the subject of EP 0 811 764 B1, which discloses a medium-speed, permanent magnet generator mounted to a single stage gearbox. The design was originally conceived by Aerodyn GmbH and has been further developed by Areva. Sometimes referred to as the “multibrid” solution, the design results in a lightweight, compact power transmission system with fewer rotating parts than most traditional drive trains.
Despite the interest in medium-speed solutions, there remains room for improvement. The highly-integrated nature of the designs makes it difficult to accommodate ancillary features like a turning gear, which is sometimes used to slowly rotate the driven components of the power transmission system during low wind speeds. Moreover, even if it is possible to incorporate a turning gear, its use may be limited. For example, significantly more torque is required to rotate the power transmission system if the turning gear is also to be used during a blade installation procedure where the hub is mounted first. At some point during such a procedure there is less than a complete set of the blades mounted to the hub, resulting in an unbalanced load about the rotational axis. Producing enough torque to rotate the unbalanced load is particularly challenging in a medium-speed power transmission system because of the smaller gear ratio in the drive train (compared to high-speed solutions). As a result, it may not be possible to incorporate a large enough motor for driving the turning gear. This limits installation options because it may be necessary to mount the blades to the hub (thereby assembling the complete wind turbine rotor) before mounting the hub to the power transmission system.